JPH06264136A - Production of thick-walled refractory steel with low yield ratio for construction use, excellent in weldability - Google Patents

Abstract

PURPOSE:To secure high strength at high temp. and to provide low yield ratio and superior weldability by specifying the rolling finishing temp. of a steel slab having specific chemical composition and the conditions of cooling, reheating, and tempering, respectively. CONSTITUTION:A slab of a steel, which has a composition consisting of 0.04-0.15% C, 0.05-0.60% Si, 0.50-1.50% Mn, 0.10-0.40% Mo, 0.005-0.060% Nb, 0.005-0.060% V, 0.005-0.030% Ti, and the balance Fe with inevitable impurities and in which the value of PCM represented by an equation us regulated to <=0.20%, is heated to >=1050 deg.C, and rolling is finished at 850 to 950 deg.C. Subsequently, the steel is subjected to accelerated cooling from a temp. not lower than Ar3 down to 400-550 deg.C at (3 to 20) deg.C/sec cooling rate. Then, the steel is reheated to a temp. between Ac1 and Ac3 and further tempered at 500-650 deg.C. If necessary, one or >=2 kinds among 0.05-0.40% Cu, 0.05-0.50% Ni, 0.10-0.40% Cr, and 0.0005-0.0050% Ca are further incorporated into the steel composition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a refractory steel, and more particularly to a low yield ratio thick wall refractory steel for construction which has a high yield strength even at a high temperature of 600 ° C. and is excellent in weldability. It is about the method.

[0002]

2. Description of the Related Art In a building structure, when a steel frame is exposed to a high temperature during a fire, its strength is lowered and the yield strength of the building is lowered.

With conventional Si-Mn-based building steels, when the temperature exceeds 350 ° C, the long-term proof strength (2/3 of the room-temperature proof strength) 217 N / mm ² required for structural members at the time of fire falls, so the temperature of the steel frame Is 350
A fireproof coating is used to prevent the construction cost and construction period from exceeding ℃, which is a hindrance.

However, the recently proposed "new fireproof design method"
However, if steel materials (fire resistant steel materials) having excellent high temperature proof strength are used, reduction or omission of the fire resistant coating has been recognized. This refractory steel is being applied to various buildings such as parking lots and intelligent buildings, and the demand for thick-walled materials is increasing.

At present, as a steel excellent in high-temperature yield strength, there is Cr-Mo steel which is widely used for boilers and pressure vessels. This steel has an excellent yield strength at 600 ° C of 217 N / mm ² or more, but its high C content causes poor weld cracking resistance and has difficulties in welding work. For this reason, there have been proposed some refractory steels which have a high yield strength at 600 ° C, excellent weldability, and can be designed and constructed in the same manner as conventional ones.

[0006] For example, the steel disclosed in Japanese Patent Application No. 1-312931 is manufactured by a controlled rolling method in which Cr, Mo and Nb are added in combination, and has excellent high temperature proof stress. Plate thickness
It is as thin as 25 mm and is not intended for thick wall. Also, Japanese Patent Application 1-
The steel disclosed in Japanese Patent No. 139329 is a mixed microstructure of ferrite and bainite obtained by accelerating cooling of steel containing a large amount of Mo from Ar ₃ or less, and suppresses the yield ratio at room temperature to a low temperature of 600 ° C. The strength of is secured. However, when accelerated cooling is performed as it is, there is a drawback that when the strip is cut by gas cutting, the residual stress is high, and thus a defective shape such as lateral bending or warpage is likely to occur.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. The chemical composition is adjusted, the rolling finish temperature is limited, and accelerated cooling is performed after the rolling finish. Reheat to a temperature range of _{1 to} Ac ₃ , then
By tempering, high strength at high temperature, low yield ratio at room temperature, and excellent weldability with excellent weldability and striation characteristics. The purpose is to provide.

[0008]

DISCLOSURE OF THE INVENTION In view of the above-mentioned problems in conventional building steels, the present invention has conducted intensive research by the present inventors, and as a result, limits chemical components and accelerates cooling after rolling.
Reheating in the Ac _{1 to} Ac ₃ temperature range-tempering ensures high strength at high temperatures, a low yield ratio at room temperature, and manufacturing of thick-walled construction fire-resistant steel with excellent weldability and slitting properties. Is to do. That is, in order to have both excellent weldability and stripping property, accelerated cooling after rolling-tempering is effective, but in this method, the microstructure becomes bainite, and Mo and Nb precipitate during tempering, so the yield ratio is high. Becomes higher. Therefore, after accelerated cooling, it is reheated to a temperature range of Ac _{1 to} Ac ₃ , and then tempered to make the microstructure a mixed structure of ferrite and bainite, thereby lowering the yield ratio and improving the weldability and stripping property. It enabled the manufacturing method of excellent low yield ratio thick wall refractory steel for construction.

The first invention is C: 0.04 to 0.15%, Si: 0.
05 to 0.60%, Mn: 0.50 to 1.50%, Mo: 0.10 to 0.40%, N
b: 0.005 to 0.060%, V: 0.005 to 0.060%, Ti: 0.005 to
A steel slab containing 0.030%, the balance of Fe and unavoidable impurities, and having a P _CM value of 0.20% or less specified by the formula (1) below is heated to a temperature of 1050 ° C or higher, 950
After completion of the rolling at a temperature range ° C., from Ar ₃ or more temperature
After accelerated cooling to a temperature of 400 to 550 ° C at a cooling rate of 3 to 20 ° C / sec, reheating to a temperature range of Ac _{1 to} Ac ₃ and further
It is a method of manufacturing low yield ratio thick wall refractory steel for construction with excellent weldability, which is tempered at a temperature of 500 to 650 ° C. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (%) …… (1)

The second invention further comprises Cu:
0.05-0.40%, Ni: 0.05-0.50%, Cr: 0.10-0.40%,
The method for producing a low yield ratio thick wall refractory steel for construction having excellent weldability according to claim 1, which contains one or more selected from Ca: 0.0005 to 0.0050%.

[0011]

The reasons for limiting the heating, rolling, accelerated cooling and heat treatment conditions in the present invention will be described below. The reason for limiting the heating temperature to 1050 ° C. or higher is to dissolve Nb and Mo, which are necessary for securing room temperature strength and high temperature strength, in steel.

Further, regarding the rolling end temperature, when the rolling end temperature is lower than 850 ° C., the yield ratio becomes high due to the grain refinement of ferrite, and the yield ratio of 80% or less cannot be obtained. Tissues are generated, and due to this, acoustic anisotropy becomes high, and the soundness of the welded part cannot be inspected because the refraction angle and the flaw detection sensitivity change in ultrasonic angle beam flaw detection. On the other hand, when the rolling end temperature exceeds 950 ° C, the austenite becomes coarse grains and the toughness deteriorates. Therefore,
Rolling end temperature is limited to the temperature range of 850-950 ℃.

In the accelerated cooling performed after the controlled rolling under the above conditions, in order to increase the amount of bainite for improving the high temperature yield strength, the time until the start of cooling is short and the starting temperature is preferably high. In particular, when the cooling start temperature is lower than Ar ₃ , ferrite is generated and the effect of increasing the high temperature strength by cooling becomes small, so the cooling start temperature is set to Ar ₃ or higher. Also,
When the cooling rate is higher than 20 ° C / sec, the strength exceeds the standard upper limit, and at a cooling rate slower than 3 ° C / sec, the strength increasing effect cannot be obtained. Therefore, the cooling rate is 3
The range is up to 20 ℃ / sec. In addition, the cooling stop temperature is 400
Limit to ~ 550 ℃. It has a cooling stop temperature of 4
This is because if the temperature is lower than 00 ° C, island martensite is generated, the toughness is significantly deteriorated, and straightening after accelerated cooling becomes difficult, while if it exceeds 550 ° C, the strength increasing effect is reduced.

After that, it is reheated to a temperature range of Ac _{1 to} Ac ₃ to partially generate ferrite, and the microstructure is made into a mixed structure of ferrite and bainite to lower the yield ratio, and then the residual stress is removed. To improve stripping characteristics
Tempering is performed in the temperature range of 500 to 650 ℃. Tempering temperature
Removal of residual stress is insufficient at less than 500 ° C, while
If the temperature exceeds 650 ° C, the room temperature strength will drop significantly. Therefore, the tempering temperature is limited to the range of 500-650 ° C.

Next, the reasons for limiting the chemical components in the present invention will be explained. C is an element that contributes to the increase in strength, but if it is less than 0.04%, it is difficult to secure the strength, and if it is added in excess of 0.15%, the weldability and toughness deteriorate. Therefore, the amount added is 0.04
The range is to 0.15%.

Si is an essential element for deoxidation,
If it is less than 0.05%, its effect is small, and if it is added in excess of 0.60%, the weldability is deteriorated. Therefore, the addition amount is set to the range of 0.05 to 0.60%.

Mn is an element necessary for ensuring the strength and toughness of steel, but if it is less than 0.50%, such an effect is small, and if it is added in excess of 1.50%, the weldability deteriorates. . Therefore, the amount added is 0.50 to 1.50.
The range is%.

Mo is an indispensable element for ensuring high temperature strength, and significantly increases the yield strength at 600 ° C.
However, if less than 0.10%, such an effect cannot be obtained, and if more than 0.40% is added, the high heat input welded joint toughness deteriorates. Therefore, the addition amount is 0.10 to 0.40.
The range is%.

Nb is an element capable of increasing the high temperature strength by precipitation hardening and transformation strengthening and improving the toughness by fine graining. However, if it is less than 0.005%, such an effect cannot be obtained, and if it is added in excess of 0.060%, the large heat input welded joint toughness deteriorates. Therefore, the amount added should be in the range of 0.005 to 0.060%.

V increases the high-temperature strength by precipitation hardening, but if it is less than 0.005%, such an effect can hardly be expected, and if it exceeds 0.060% in excess, the weldability deteriorates. Therefore, the amount added is 0.005-0.060%.
The range is.

Ti is an element which suppresses coarsening of austenite grains and also serves as a nucleation site for ferrite, which is effective for grain refinement. However, if less than 0.005%, such effect cannot be exhibited, and if more than 0.030% is added, the toughness of the base material deteriorates. Therefore, the amount added is in the range of 0.005 to 0.030%.

In the second invention of the present invention, one or more of Cu, Ni, Cr and Ca can be added in addition to the above elements, if necessary.

Cu is an element effective for increasing strength by solid solution strengthening, but if it is less than 0.05%, such an effect is small,
If added in excess of 0.50%, hot workability and weldability are impaired. Therefore, the addition amount is set to the range of 0.05 to 0.50%.

Ni is an element effective for improving toughness,
If it is less than 0.05%, such an effect cannot be obtained. Also, 0.
Even if added in excess of 50%, such an effect is saturated and it is economically wasteful. Therefore, the amount added is 0.05-0.
The range is 50%.

Cr is an element effective in improving the high temperature strength, but if it is less than 0.10%, such an effect cannot be expected, and
If added in excess of 60%, the weldability will deteriorate. Therefore, the addition amount is set to 0.10 to 0.60%.

[0026] Ca is an element that improves the properties in the plate thickness direction with a trace amount, but if it is less than 0.0005%, there is no such effect,
On the other hand, when added in excess of 0.0050%, such effects are saturated and large inclusions are generated, so that ultrasonic defects are likely to occur. Therefore, the amount added is 0.0005-
The range is 0.0050%.

Further, in the present invention, P shown in the formula (1) is used for the purpose of omitting preheating which is carried out to prevent cold cracking during welding.
_CM (welding crack susceptibility index) is limited to 0.20% or less.

By using the above conditions, a high yield strength is ensured at a high temperature, a yield ratio at room temperature is low, and a method for producing a thick-walled construction refractory steel having excellent weldability and slitting properties is provided. Can be provided. That is, appropriate component design and controlled rolling and subsequent accelerated cooling-Ac ₁ ~
By applying a combination of reheating and tempering in the Ac ₃ temperature range and controlling the steel structure to be a ferrite and bainite structure, the low yield ratio thick wall refractory steel for construction with excellent weldability, which is the object of the present invention, is obtained. It can be manufactured.

[0029]

EXAMPLES The present invention will be described below with reference to examples. Example 1 The test steel sheet is a steel piece having the chemical composition shown in Table 1 and finished to a thickness of 70 mm according to the heating, rolling, accelerated cooling, and heat treatment conditions shown in Table 2. Test pieces were taken from these steel sheets and subjected to a room temperature tensile test, a Charpy impact test, a high temperature tensile test at 600 ° C and a maximum hardness test. The results are also shown in Table 2. The highest hardness test is JIS Z 31.
It went according to 01.

Table 1 shows the methods A to H of the present invention and comparative examples I to N.
Table 2 shows the chemical components of the above, heating, rolling, accelerated cooling, heat treatment conditions, tensile properties, impact properties, high temperature properties and weldability.

As is apparent from Table 2, the present invention methods A to H
Has a high temperature proof stress of 217 N / mm ² or more at 600 ° C, a maximum hardness of less than HV300, and low weld hardenability. In addition, the yield ratio fully satisfies the requirement of 80% or less required for building steel, and the fracture surface transition temperature in the Charpy impact test is also good at -40 ° C or less.

[0032]

[Table 1]

[0033]

[Table 2]

On the other hand, Comparative Example I, since the C and P _CM is out to increase the limiting scope of the present invention, is poor base metal toughness and weldability. In Comparative Example J, Mo is out of the limited range of the present invention, so that the base material toughness and weldability are poor.
In Comparative Example K, the base material toughness is poor because Ti is not added. Further, in Comparative Examples L and M, since the former is not added with Mo and the latter is not added with Nb, the yield strength at 600 ° C. is low. Further, Comparative Example N has a low yield strength at 600 ° C. because V is not added.

Example 2 The test steel sheet was finished to a thickness of 70 mm according to the heating, rolling, accelerated cooling and heat treatment conditions shown in Table 3. Test pieces were taken from these steel sheets and subjected to a room temperature tensile test, a Charpy impact test, and a high temperature tensile test at 600 ° C. The results are also shown in Table 3 below. The chemical components are the method A of the present invention in Table 1.
Is the same as.

In the method A1 to A5 of the present invention, the heating temperature is 1050-1250.
℃, rolling end temperature is 860-930 ℃, cooling start temperature is Ar ₃
Above, the cooling stop temperature is 420 ~ 520 ℃, the cooling rate is 5 ~ 10
C./sec, reheating temperature at Ac _{1 to} Ac ₃ is 780 to 820 ° C., tempering temperature is 600 to 630 ° C., and room temperature strength, yield ratio, fracture surface transition temperature and high temperature proof stress are all good.

[0037]

[Table 3]

On the other hand, in Comparative Example A6, since the heating temperature is 1000 ° C. and Nb is not sufficiently dissolved in solid solution, the room temperature strength and the high temperature proof stress are low. Comparative Example A7 has a high high-temperature yield strength because it is not reheated from Ac _{1 to} Ac ₃ , but has a yield ratio at room temperature of 80% or more and is not suitable for construction. Comparative Example A8
Since the rolling end temperature is low at 830 ° C, the ferrite becomes fine grains and the yield ratio at room temperature exceeds 80%. Further, in Comparative Example A9, the rolling end temperature was as high as 960 ° C., so that the austenite became coarse particles and the fracture surface transition temperature was high. Comparative Example A10 is
Since the cooling start temperature is Ar ₃ or less, bainite is less generated, and room temperature strength and high temperature proof stress are low. In Comparative Example A11, the cooling stop temperature was as high as 570 ° C., so the strength increasing effect was small, and the room temperature strength and high temperature proof stress were low. Furthermore, Comparative Example A1
No. 2 has a low cooling rate of 2 ° C / sec, so its room temperature strength and high temperature yield strength are low. Further, in Comparative Example A13, since the cooling stop temperature is as low as 320 ° C., island-shaped martensite is generated,
High fracture transition temperature.

Although the above embodiment relates to a method for manufacturing a thick steel plate, the present invention is applicable to other steel products, such as bar steel,
It goes without saying that it can also be applied to the production of shaped steel.

[0040]

As described above, according to the present invention, the slab of which the chemical composition is adjusted and the P _CM is regulated to 0.20% or less is 1050
After heating to a temperature of ℃ or more and finishing the rolling in a temperature range of 850 to 950 ℃, after accelerated cooling from a temperature of Ar ₃ or more to a temperature of 400 to 550 ℃ at a cooling rate of 3 to 20 ℃ / sec, Ac ₁ ~
A method for producing a low yield ratio thick wall refractory steel for construction having excellent weldability, which comprises reheating to a temperature range of Ac ₃ and further tempering at a temperature of 500 to 650 ° C. According to the present invention, it is high at high temperature. It is possible to obtain a thick wall refractory steel with a low yield ratio for construction that secures strength, has a low yield ratio at room temperature, and has excellent weldability and striation characteristics and excellent weldability. Therefore, the fire-resistant steel for construction according to the method of the present invention can greatly reduce or omit the fire-resistant coating that has been conventionally required, and further enhance the safety of the structure from the viewpoint of earthquake resistance, and the construction efficiency. Can be improved.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeo Okano 1 Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kadogawa Works Kakogawa Steel Works

Claims (2)

[Claims] 1. C: 0.04 to 0.15%, Si: 0.05 to 0.60%, M
n: 0.50 to 1.50%, Mo: 0.10 to 0.40%, Nb: 0.005 to 0.060
%, V: 0.005 to 0.060%, Ti: 0.005 to 0.030%, the balance consisting of Fe and unavoidable impurities, and
A slab with a P _CM value of 0.20% or less specified by equation (1)
After heating to a temperature of 1050 ° C or higher and finishing rolling in the temperature range of 850 to 950 ° C, after accelerated cooling from a temperature of Ar ₃ or higher to a temperature of 400 to 550 ° C at a cooling rate of 3 to 20 ° C / sec. , A
A method for producing a low yield ratio thick wall refractory steel for construction with excellent weldability, which comprises reheating to a temperature range of c _{1 to} Ac ₃ and further tempering at a temperature of 500 to 650 ° C. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (%) …… (1) 2. As a chemical component, Cu: 0.05 to 0.40 is further added.
%, Ni: 0.05 to 0.50%, Cr: 0.10 to 0.40%, Ca: 0.0005
The method for producing a low yield ratio thick wall refractory steel for construction having excellent weldability according to claim 1, containing one or more selected from the range of 0.0050% to 0.0050%.